Metallic compositions which that are known to be capable of undergoing a reversible transformation from the austenitic state to the martensitic state include unalloyed metals but this phenomenon is most commonly exhibited by alloys. Such alloys include, for example, those disclosed in U.S. Pat. Nos. 3,012,882, 3,174,851; 3,351,463; 3,567,523; 3,753,700; and 3,759,552, Belgian Pat. No. 703,649 and in British Pat. Nos. 22372/69, 55481/69, 55482/69, 55969/69 and 53734/70 (Now Brit. Pat. Nos. 1,315,652; 1,315,653; 1,346,046 and 1,346,047) in the name of the Fulmer Research Institute. The disclosure of each of the aforementioned patents and applications is incorporated herein by reference.
Such alloys are disclosed in NASA Publication SP110, "55-Nitinol-the alloy with a memory, etc." (U.S. Government Printing Office, Washington, D.C., 1972), N. Nakanishi et al, Scripta Metallurgica 5, 433-440 (Pergamon Press 1971), the disclosures of which are likewise incorporated herein by reference.
These, and some other alloys, have in common the feature of undergoing a shear transformation on cooling from a relatively high temperature (austenitic) state to a relatively low temperature (or martensitic) state. If an article made of such an alloy is deformed when in its martensitic state it will remain so deformed. If it is heated to return it to a temperature at which it is austenitic, it will tend to return to its undeformed state. The transition from one state to the other, in each direction, takes place over a temperature range. The temperature at which martensite starts to form on cooling is designated M.sub.s while the temperature at which this process is complete is designated M.sub.f, each of these temperatures being those achieved at high, e.g., 100.degree. C. per minute, rates of change of temperature of the sample. Similarly, the temperature of the beginning and end of the transformation to austenite are designated A.sub.s and A.sub.f respectively. Generally, M.sub.f is a lower temperature than A.sub.s, M.sub.s is a lower temperature than A.sub.f, and M.sub.s can be lower, equal to or higher than A.sub.s, for a given alloy depending on composition and thermomechanical history. The transformation from one form to the other may be followed by measuring one of a number of physical properties of the material in addition to the reversal of deformation described above, for example, its electrical resistivity, which shows an anomaly as the transformations take place. If graphs of resistivity-v-temperature or strain-v-temperature are plotted, a line joining the points M.sub.s, M.sub.f, A.sub.s, A.sub.f and back to M.sub.s forms a loop termed the hysteresis loop (see Diagram 1, below). For many materials M.sub.s and A.sub.s are at approximately the same temperature. ##STR1##
One particularly useful alloy possessing heat recoverability or shape memory is the intermetallic compound TiNi, as described in U.S. Pat. No. 3,174,851. The temperature at which deformed objects of heat recoverable alloys return to their original shape depends on the alloy composition as disclosed in British Pat. No. 1,202,404 and U.S. Pat. No. 3,753,700, e.g., the recovery of original shape can be made to occur below, at, or above room temperature.
In certain commercial applications employing heat recoverable alloys, it is desirable that A.sub.s be at a higher temperature than M.sub.s, for the following reason. Many articles constructed from such alloys are provided to users in deformed condition and in the martensitic state. For example, couplings for hydraulic components, as disclosed in U.S. Pat. applications Ser. No. 852,722 filed Aug. 25, l969 and No. 51809 filed July 2, 1970 (British Nos. 1,327,441 and 1,327,442), are sold in a deformed (i.e., an expanded) state. The customer places the expanded coupling over the components (for example, the ends of hydraulic pipe lines) to be joined and raises the temperature of the coupling. As its temperature reaches the austenitic transformation range, the coupling returns, or attempts to return, to its original configuration, and shrinks onto the components to be joined. Because it is necessary that the coupling remain in its austenitic state during use (for example, to avoid the stress relaxation which occurs during the martensitic transformation and because its mechanical properties are superior in the austenitic state), the M.sub.s of the material is chosen so as to be below the lowest temperature which it may possibly reach in service. Thus, after recovery, during service the material will remain at all times in the austenitic state. For this reason, once deformed it has to be kept in, for example, liquid nitrogen until it is used. If, however, the A.sub.s (A.sub.s as used herein, means that temperature which marks the beginning of a continuous sigmoidal transition as plotted on a strain vs. temperature graph, to the austenitic state of all the martensite capable of undergoing that transformation) could be raised, if only temporarily for one heating cycle, without a corresponding rise in the M.sub.s then the expanded coupling could be maintained at a higher and more convenient temperature. The advantage this would provide is an obvious one. For example, if the A.sub.s of the alloy from which it is made could be raised sufficiently to allow the coupling to be handled at ambient temperature without recovery occuring, it would be possible to avoid the problems and expense associated with prolonged storage of the heat recoverable coupling that must be kept in liquid nitrogen after deformation.
In copending and commonly assigned U.S. application "Heat Treating Method", Ser. No. 550,847, filed on even date herewith (Lyon & Lyon Docket No. 145/201) as a C.I.P. of application Ser. No. 417,067, filed Nov. 19, 1973, abandoned the disclosure of both of which are incorporated by reference, we have described a method by which the A.sub.s of certain metallic compositions can be raised for one heating cycle. This method comprises first lowering the temperature of the composition from that at which it exists in the austenitic state to below its M.sub.f temperature. Then the composition is heated to a temperature at which normally it would exist wholly in the austenitic state, i.e. above the A.sub.f temperature. However, the transformation from martensite to austenite does not occur if the heating rate selected is a "slow" one. The definition of a "slow" heating rate is fully set forth in said copending application. Suffice it to say that it can vary depending upon the nature of the metallic composition but is easily determined by one skilled in the art having the benefit of said application.
If the composition is cooled after slow heating is complete and subsequently reheated at a rapid rate it does not begin to undergo a martensite to austenite transformation until the approximate temperature at which slow heating was terminated is reached. More importantly, if an article was made from the composition and deformed while in the martensite state either prior to, or after, slow heating is terminated, it will not begin to undergo recovery to the form in which it existed in the austenitic state until it reaches approximately the temperature at which slow heating was terminated. We refer to this process as "thermal preconditioning."
Disclosed by reference in our other copending application cited hereinabove is our discovery that the tendency of some metallic compositions to lose martensite-austenite reversibility, e.g. particularly as occurs with some compositions with M.sub.s of 0.degree. C. or higher, can be inhibited. This method comprises "aging" the composition by holding it an an elevated temperature, typically 50.degree.-150.degree. C., in which it exists in the austenitic state prior to transforming it to the martensitic state. The aging temperature and the holding time required to inhibit loss of this reversibility can vary according to the nature of the composition but can be readily determined by those skilled in the art having the benefit of the disclosure in said application.
As a result of our previous discoveries, it has been found possible to prepare useful heat recoverable articles from metallic compositions which as a result of our treatment have a significantly reduced tendency to lose martensite-austenite reversibility and also have an elevated A.sub.s temperature. However, notwithstanding the many advantages that our discoveries have provided the art, in order to elevate the A.sub.s temperature for metallic compositions it is necessary that equipment capable of providing a controlled "slow" heating rate be employed. Furthermore, it is necessary that some preliminary investigation be done with compositions other than those specifically described by us in order to determine the optimum slow heating rate. Finally, the "slow" heating rate necessary to avoid the onset of recovery may necessitate an undesirably long preconditioning period to achieve the desired A.sub.s. Therefore, it would be advantageous to have a method by which an elevated A.sub.s can be imparted to metallic compositions capable of undergoing a reversible transformation between an austenitic state and a martensitic state that does not suffer these limitations.
Accordingly, it is an object of this invention to provide an improved method for imparting an elevated A.sub.s for at least one heating cycle to metallic compositions that undergo a reversible transformation between an austenitic state and a martensitic state. It is yet another object of this invention to provide novel metallic compositions that have such as elevated A.sub.s temperature.